1. Field of the Invention
The present invention relates to a device for burning a fuel/oxidant mixture in a strongly exothermic reaction, the device including a reactor with a combustion chamber containing at least one first porous material and at least one second porous material in separate zones, whereby the zones are designed in such a way that an exothermic reaction can only occur in the second zone, and with one or more feed lines for the fuel and for the oxidant.
2. Description of the Related Art
Documents DE 43 22 109 C2 and DE 199 39 951 C2 describe devices which are designed as so-called porous burners. The combustible gas mixture initially flows through one region, which is referred to as zone A and which has sufficiently small effective pore diameters so as to not allow stationary flame spreading. In other words, the first porous zone is operationally similar to a flame arrester. The following actual combustion region, which is referred to as zone C, has greater pore sizes which are large enough to permit a stationary burning. A critical Péclet number of Pe>65 is cited in technical literature (for example Babkin, et al., in “Combustion and Flame, Vol. 89, pages 182-190, 1991) as criterion for the spreading of flames in the interior of a porous matrix.
Materials such as aluminum oxide, zircon oxide, silicon carbide, etc., which in addition to high temperature resistance, also possess sufficient corrosion resistance, can be used as porous combustion chamber filling in porous reactors for chemical industrial plants. To produce a porous combustion chamber, bulk material of temperature resistant ceramic balls, saddle packing or similar bodies are used, as are preferably used for example as random packing for thermal separation processes. Bulk materials are preferred because they allow easy clean-up of deposits, for example of salt residues which occur in hydrogen chloride synthesis, originating from the combustion gases. According to DE 43 22 109 C, in porous burners, zones of different pore structure or respectively pore size are arranged in order to produce hydrogen chloride zones. This is done by using filler bodies of different sizes for zones A and C. In addition, structured packing and foams may be used in zones A and B.
According to document DE 199 39 951 C2, an additional support grate can be arranged between the porous structures formed by filler bodies in the two zones and having different pore sizes. The support grate prevents the discharge of smaller sized filler bodies from zone A into the inter-spaces of the larger filler bodies in zone C. In burners where gases do not exit vertically, or in an upward direction, another gas-permeable grate is arranged at the gas exit from zone C which closes the combustion chamber. As a result, it is possible to arrange the reactor in any random position despite the loose bulk of filling bodies in the combustion chamber.
The porous reaction chamber is preferably encased by a corrosion resistant cooled wall which consists, for example, of artificial resin-impregnated graphite. Cooling can be effected through cooling water, air or by the combustion gases themselves. Between the cooled wall and the combustion chamber is then preferably located an insulating intermediate layer of high temperature resistant, corrosion resistant and thermally insulating materials, which prevent loss of heat and which ensure that the desired combustion chamber temperature prevails at each location in the combustion chamber. According to the document DE 199 39 951 C2, this heavy insulation permits an almost adiabatic process control without any temperature influence on the combustion process as a result of the cooled wall. The adiabatic process control permits, for example, simple scale-up of such chemical reactors since heat transport properties are irrelevant to the cooled walls and the entire process in a flow direction can be regarded as almost one-dimensional.
In a porous reactor the reaction is conducted inside a porous matrix consisting of temperature resistant material. In deviating from conventional reactor devices it is not necessary to arrange the reactor in a voluminous combustion chamber or to locate such downstream. From the reactor itself, the hot reaction products flow without direct flame formation. In DE 43 22 109 C2 it is suggested to use a clearly lower Péclet number for the first zone, and a clearly higher Péclet number for the combustion zone than the critical Péclet number of Pe=65.
When the porous reactor is being ignited, the combustion stabilizes at the interface between the two zones. Due to the smaller pore dimensions in the first zone no combustion occurs in this region in a stationary state, only pre-heating of the gas mixture. This characteristic also fulfills the most stringent safety regulations with regard to the danger of flashback in chemical plants.
Due to the excellent heat transfer between gas and solid phases inside the porous matrix, they are in approximate thermal balance. The approximate thermal balance between gas and solid phases and the intensive blending inside the pore body essentially causes the disappearance of free flames in the combustion zone which is equipped with larger pores. The burning process is now performed in an extended reaction area which can be classified as a combustion reactor, rather than combustion chamber with free flames. According to document DE 199 39 951 C2 the pre-mixing chamber is part of and a safety relevant component of the described device.
A disadvantage of the existing construction forms exists in the locally restricted temperature acquisition by means of thermo-elements in the reaction zone. A further disadvantage of known porous reactors whose porous layers are made up of bulk material consists in that the bulk material bodies are carried along by the gas flow in the case of a higher or suddenly increased gas throughput, thereby leading to changes in the bulk material density as well as in the Péclet number. A stable process control under greatly changing gas throughput conditions, especially for controlled burning of larger volumes of halogenated gases during abnormal occurrences, is possible only to a very limited extent.
What is needed in the art is a reactor which permits the exothermic chemical reaction of a fuel/oxidant mixture while providing a stable process control under changing gas throughput conditions.